Mathematics for the interested outsider

The Fatou-Lebesgue Theorem

Now we turn to the Fatou-Lebesgue theorem. Let be a sequence of integrable functions (this time we do not assume they are non-negative) and be some other function which dominates this sequence in absolute value. That is, we have a.e. for all . We define the functions

These two functions are integrable, and we have the sequence of inequalities

Again, this is often stated for a sequence of measurable functions, but the dominated convergence theorem allows us to immediately move to the integrable case. In fact, if the sequence converges pointwise a.e., then a.e. and the inequality collapses and gives us exactly the dominated convergence theorem back again.

Since dominates the sequence , the sequence will be non-negative. Fatou’s lemma then tells us that

Cancelling the integral of we find the first asserted inequality. The second one is true by the definition of limits inferior and superior. The third one is essentially the same as the first, only using the non-negative sequence .

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[…] Then you can prove the monotone convergence theorem, followed by Fatou’s lemma, and then the Fatou-Lebesgue theorem, which leads to dominated convergence theorem, and we’re pretty much back where we […]

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This is mainly an expository blath, with occasional high-level excursions, humorous observations, rants, and musings. The main-line exposition should be accessible to the “Generally Interested Lay Audience”, as long as you trace the links back towards the basics. Check the sidebar for specific topics (under “Categories”).

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